The present invention relates to a print head of a wire dot printer, and more particularly to a wire dot print head in which a pair of guide nose halves are made to abut to form a guide nose.
An example of a print head used in a wire dot printer is a spring-charged print head shown in FIG. 1 which is a partial cross section. The dot print head 309 has plate springs 393 and armatures 394 between a guide nose 391 and a head printed circuit board 392. Print wires 395 provided at the free ends of the armatures 394 extend through wire guides 396 and project from guide holes 397 provided in the tip guide 391a of the guide nose 391.
That is, the print wires 395 are surrounded or enveloped by the guide nose 391, and the wire guides 396 disposed inside the guide nose 391 limit movement of the print wires 395 and restrain vibrations.
A permanent magnet 399 and electromagnets 398 which are provided below the guide nose 391, and yokes 394a form magnetic circuits. When the electromagnets 398 are not energized, the armatures 394 are attracted toward the electromagnets 398 due to the magnetic flux from the permanent magnet 399. When the electromagnets 398 are energized, the magnetic flux from the electromagnets cancel the magnetic flux from the permanent magnet and the armatures 394 are released so that the print wires 395 are projected from the guide holes 397 by virtue of the resilient reactive force of the plate springs 393 to press the print wires 395 against an ink ribbon IR and a print paper PP on a platen PL.
When the dot print head 309 of the above structure is assembled, a wire assembly having print wires 395 inserted through the wire guides 396 is assembled from below (as seen in FIG. 1) the guide nose 391 and the tips of the print wires 395 are aligned with the guide holes 397. For this purpose, an air gap AG is needed to permit the wire guide 396 to fit in the guide nose 391.
However, because of the presence of the air gap AG, carbon particles or the like that have entered through the guide holes 397 are accumulated inside the guide nose 391 and cause oxidation of the print wires 395.
Moreover, noise, i.e. contact-slide noise, generated when the print wires 395 slide against the wire guide 396 is loud because the air gap AG functions as an echoing chamber.
Another prior-art print head is shown in a sectional view of FIG. 2. As shown, it comprises plate springs 471 and armatures 472 which are provided between a guide nose 470 and a head printed circuit board 480. The print wires 473 fixed to the free ends of the armatures 472 are made to project from guide holes 474a of the tip guide 174.
An intermediate guide 476 and a guide felt 477 provided in the cavity in the guide nose 470 support the print wires 473 and serves to prevent vibration. In a drive part provided below the guide nose 470, a magnetic circuit is formed of electromagnets 478, a permanent magnet 479 and yokes. By virtue of this magnetic circuit, the armatures are attracted, and by virtue of the resilient reactive force of the plate springs 471, the print wires 473 are projected from the guide holes 474a to press an ink ribbon IR and a paper PP onto a platen PL. Printing is thereby accomplished.
When the wire dot print head 409 of the above structure is assembled, as shown in the exploded oblique view of FIG. 3, print wires 473 on a wire assembly 481 are assembled from below the guide nose 470 and the tips 473a of the print wires 473 are fitted in the guide holes 474a of the tip guide 474. Then, a guide felt 477, for preventing vibration of the print wires 473 and the like, is inserted through an insertion opening 490 provided on the side of the guide nose 470 into a cavity 475.
The print wires 473 on the spring assembly 481 are normally thin and not associated with support members so that although they stand by themselves they are easy to vibrate and therefore it is difficult to align them with and insert them in the holes 476a of the intermediate guide 476. Moreover, even when they are fitted in the holes 476a, the tips 473a of the print wires again have to be aligned with the holes 474a in the tip guide 474. It is thus laborious to fit the print wires in the guide holes 474a and the holes 476a. On the other hand, the insertion opening 490 provided at the side of the guide nose 470 is open even after the guide felt 477 is inserted, so that the contact-slide noise and the like of the print wires 173, which is generated when the wire dot print head is driven, leaks out of the insertion opening 490 causing a high noise, level.